The accumulation of ice on various surfaces of an aircraft can produce disastrous results. Accordingly, it is important for a pilot to know when ice starts to appear and to take measures to avoid serious consequences. For example, a pilot may turn on surface heaters, make changes in speed or elevation, changes in the angle of attack and/or seek the nearest airport and land before the problem becomes critical.
Over the years there have been a number of approaches for detecting icing on an aircraft during flight. For example, a U.S. Pat. No. 3,996,787 of Edgington discloses an Apparatus for Indicating Ice Accretion. As disclosed therein such apparatus includes an endless test surface upon which ice forms and a gas flow passage terminating adjacent to the test surface in an aperture which is presented to and spaced by a known gap from the test surface. A flow of gas is generated in the passage toward the aperture and a drive mechanism drives the test surface past the aperture at a constant rate. An ice removal device is operable to remove ice from the test surface after the surface has moved past the aperture and a sensor detects variations in the flow or pressure of the gas in the passage. Such variations arise from the effect on gas flow of restriction of the gas by ice forming on the surface. An instrument is associated with the sensor to provides an indication of the extent of the variation in flow or pressure and provide an indication of the rate at which ice is being accumulated on the test surface.
A more recent approach for determining ice formation on a aircraft's surface is disclosed in a co-pending U.S. patent application of Paul Levine entitled Optical Systems and Element for Detecting Ice and Water, Ser. No. 11/168,363 filed on Jun. 29, 2005 and assigned to Safe Flight Instrument Corporation, the same Assignee of the present application. As disclosed therein, an optical system for detecting ice and water on the surface of an aircraft includes an elongated transparent optical element having first and second end portions. A light source and light detector are disposed in one end of the optical element and a reflective surface is disposed in the opposite end portion. The reflective surface defines a critical angle and reflects light from the light source to the light detector when the critical angle is in contact with air and refracts light toward the external environment when the reflective surface is in contact with ice or water. The system may also incorporate an optical element wherein the reflective surface includes a continuous array of convex elements extending outwardly from and across one end of the optical element and wherein each of the convex elements defines a critical angle. The Levine application is incorporated herein in its entirety by reference.
Not withstanding the above, it is presently believed that there is a need and a potential market for an improved ice detector and system and method in accordance with the present invention. Such systems provide an indication of initial ice formation and subsequently an indication of the rate of ice formation or thickness of accumulated ice. In addition to the above, the systems in accordance with the present invention provide periodic or intermittent tests and an audible warning as soon as ice formation is detected. Then, if the thickness of the ice increases the system provides a second audible warning distinguishable from the first signal that the thickness of the ice or rate of ice deposit has increased. Then if the thickness increases again a third warning may be provided. In one embodiment of the invention, ice detection and accretion system is combined with a stall warning system and increases the margin in the stall warning system to accommodate for the ice formed on the aircraft. It is also believed that the system in accordance with the present invention is durable, reliable, can be manufactured at a competitive cost and easily serviced.